1. Field of the Invention
This invention relates to a shell boiler of the type comprising an outer continuous shell within which is contained the steam and/or water space of the boiler, a furnace tube containing a furnace grate, and a plurality of smoke tubes for passage therethrough of products of combustion of the furnace, the boiler having apparatus for feeding solid fuel to the furnace grate downwardly through the steam and/or water space of the boiler.
2. Summary of the Invention
An object of the present invention is to provide a new and improved boiler of the type described.
According to the present invention, we provide a boiler of the kind specified wherein the apparatus for feeding solid fuel to the furnace grate comprises a means for classifying and separating fine particles of fuel having a particle size below a predetermined size, from the remainder of the fuel and feeding the fine particles into the furnace tube at a different location to that at which the remainder of the fuel is introduced into the furnace tube.
The boiler may be provided with a grit arrestor arranged to extract grit entrained in the products of combustion of the boiler and means to feed the extracted grit into the furnace tube, and wherein said fine particles of fuel are fed into the furnace tube with the extracted grit.
The grit arrestor may be located at a rear end of the boiler and fine particles may be fed thereto and both fine particles and the extracted grit fed from the grit arrestor into the furnace tube.
The fine particles, or fine particles and extracted grit, may be introduced into the furnace tube in a direction at any angle from a right angle to 180.degree. to the horizontal vector of the direction of flow of the products of combustion from the furnace tube.
The fuel feed means may comprise a first conduit extending downwardly from the fuel classifying and separating means through the roof of the furnace tube, pneumatic transport means to provide a flow of transportair to feed fuel along a second conduit to the fuel classifying and separating device and a third conduit extending from the fuel classifying and separating device to a fines-air separating means for separating the fine particles from the transport air.
When the furnace is provided with a grit arrestor, the fines-air separating means may be connected thereto so that the fines are fed to the grit arrestor while the transport air is fed to the intake side of a fan which provides said transport air for the fuel.
The fine particles or fine particles and grit may be fed into the furnace tube by a second pneumatic transport means the transport air of which is provided by a main combustion air fan of the boiler.
The grit arrestor may be provided with a venturi device so that said second transport air withdraws grit and fine particles from the grit arrestor by venturi action and transports them to the furnace tube.